With the growth of the electronics industries over the last two decades the use of electrodeposited copper foil in integrated circuit boards, which are used in computers and other electronic products, has assumed increasing importance. The copper foil used for such purposes must be of very high quality, of uniform thickness, smooth and free from surface imperfections.
The electrodeposition of a metal on a rotating drum cathode from a metal-containing electrolyte to produce a thin metal foil on the surface of the drum has been in use for many years. For example, U.S. Pat. No. 3,674,656 discloses an electrochemical process for the manufacture of copper foil for use in the preparation of printed circuit boards. The process uses a drum cathode which rotates partially immersed in a body of copper sulfate electrolyte adjacent to a pair of concentric anodes. Typically, the anodes, which are insoluble, are made of lead, lead-antimony, platinized titanium or oxides of iridium and ruthenium. The top, or outer, surface of the drum is typically made of stainless steel, titanium, or stainless steel plated with chromium. Often times, the drum is constructed of, for example, a titanium top sheet, or cylinder, over an underlying, or supporting, base cylinder of a less expensive metal in order to reduce costs.
As the drum cathode rotates in the electrolyte, an electrodeposit of copper forms on the outer surface of the drum. The electrodeposited copper is stripped from the surface of the rotating drum in the form of a thin foil. In such a process, the amperage used directly determines the amount of copper electrodeposited on the cathode.
In the past, much effort has gone into improving the equipment used for the production of electrodeposited thin metallic foils, especially copper foil. Much of this effort concentrated on the development of improved-performance drum cathodes. Since the side of the foil next to the drum's outer surface replicates the surface on which it forms, it is important that the outer surface of the drum be smooth and free from cracks and the like. Therefore, drums capable of maintaining a smooth surface have been sought. This is because unless one starts with a raw foil (as it comes off the drum) of suitable quality, no treatment, no matter how good, can transform the foil into a satisfactory product.
The primary features of a good drum with a good outer, or plating, surface are as follows:
1. Good corrosion resistance, especially on the plating surface; PA1 2. Good surface finish with good adhesion and foil stripping characteristics; PA1 3. Capability of carrying high electric current (for high output); PA1 4. Efficiency in making porosity-free and defect-free foil; PA1 5. Ability to produce uniform thickness of foil across the complete drum width; PA1 6. Low labor and low cost maintenance; and PA1 7. Other requirements such as availability of components, machinability, non-toxicity, cost effectiveness, and the like.
These requirements, and particularly those involving the drum surface, lead inevitably to the development of a titanium drum. The copper foil industry initially utilized a lead drum. The corrosion rate of the drum surface in sulfuric acid was, however, so high that the drum surface had to be "brushed" continuously. In order to solve this corrosion problem, foil manufacturers experimented with and used stainless steels, chromium, chromium-plated stainless steels, titanium and its alloys, and, less commonly, zirconium in the drum surface. Titanium has received the most attention and acceptance and is disclosed as the cathode material in U.S. Pat. No. 2,646,396. Indeed, titanium comes close to meeting all the requirements of an ideal drum-surface material. It has excellent corrosion resistance to sulfuric acid in the relevant range of acid concentration (10-20%) and temperature (120.degree.-200.degree. F.), is commercially available, machineable, and non-toxic, has good copper foil stripping/adhesion properties, requires low maintenance, produces a non-porous defect-free foil with a fine matte surface finish, and finally, its electrical conductivity, while not excellent, is better than that of stainless steel.
Despite this impressive set of desirable "material" properties, titanium drums in the electrodeposited copper foil industry were found to suffer from serious problems. "Hot spots" would appear on the drum surface and within days or weeks cause it to fail and severely limit its productive life. A detailed account of the history of titanium drum development and "hot spot" problem is disclosed in U.S. Pat. No. 4,240,894.
Another serious but related problem with titanium drums has been a limitation on the current density they can carry, the maximum current density being about 350 amperes per square foot. This, for a given drum size, limited the total current the drum could carry which, in turn, placed a corresponding limit on the drum's output capacity of metal foil as expressed in weight per unit of time. When this current density was exceeded the drum was found to become readily prone to the development of hot spots and, consequently, it had a short life.
During the course of investigating the formation of hot spots and consequent premature failure of prior art titanium drum cathodes, I have developed the following understanding of this phenomenon, which, in turn, has permitted me to develop the present invention as a solution to the above problems which have plagued the industry for many years.
It has been the practice in industry to build titanium drum cathodes as a cylindrical underdrum formed of a less expensive metal with a concentric titanium top sheet, or cylinder, directly over it. In such a drum cathode, the electrical and thermal connections, their stability and their cross sections are a major focus of the present invention.
Previously, the causes of hot spots were not understood and, therefore, the prior art proposed solutions to this problem which were not effective to eliminate the formation of hot spots during use. It was perceived that very high mechanical forces coupling the titanium top cylinder, or sheet, of the drum to the underdrum, or base sheet, directly below it would prevent hot spots from developing. To obtain these high coupling forces the underdrum was made of a material that combined high electrical conductivity with ductility and a high linear temperature coefficient of expansion (LTCE) compared to that of titanium. The titanium top cylinder had an inside diameter slightly smaller than the outside diameter of the underdrum (supporting cylinder), and the top cylinder was heated and shrink-fitted over the underdrum. This method is disclosed in U.S. Pat. No. 3,461,046 and, with some modifications, in U.S. Pat. No. 4,240,894.
The above shrink fitting method is, in principle, a scheme that relies on mechanical force to make electrical contact between two surfaces. There is, however, a distinction between an "actual" and an apparent contact surface. A book on a table gives a simple illustration of the two quantities. The apparent contact surface, in this case, would be the area defined by the product of the width of the book's cover multiplied by its length. If the book's cover happens to be 9".times.12", the apparent contact area between the book and the table would be 108 square inches. The two surfaces in contact are, however, not perfectly flat and smooth. Therefore, the actual contact surface is made up of a collection of very small spots scattered over the apparent contact surface and which, when added all together, represent a small fraction, perhaps one percent, of the apparent contact area. The exact value of this actual contact surface depends on the hardness of the book cover, the hardness of the table top on which it rests, and the force pressing these two objects against each other. If both the book and the table were infinitely hard, they could touch on three small spots. However, since all materials are deformable to some degree, one finds that as the loading force is increased on the book, the initial contact spots become larger and new contact spots come into being. Up to a certain limit, when the loading force on the book is removed, both surfaces are restored to their initial condition and no permanent deformation has taken place on either surface. The range of mechanical loading force through which no permanent deformation occurs, is called the "elastic range". If the loading force is increased beyond this range, permanent deformation of one or both contact surfaces occurs and the deformation is said to be of the "plastic type".
When two "apparently" flat bodies come into contact under some mechanical load, some of the actual contact spots undergo elastic, and others plastic deformation. This is because on the scale of these contact spots, these nominally flat contact surfaces are neither flat nor even and, consequently, some spots bear more load than others, putting the more loaded ones in the plastic range.
The above-described principle also applies to an equivalent one foot square segment of a titanium top sheet shrink-fitted on its supporting portion of the base drum. Therein, the apparent contact interface of one square foot exists between these two metallic members, but there is a much smaller actual contact surface which bears all the loading force. This actual contact surface is made up of a collection of small isolated peaks, or contact spots, some which are elastically deformed and others of which are deformed plastically. A contact spot is clearly established when a peak of one member interfaces with the other surface. If an electric current were to pass through such an interface, it would see an overwhelmingly large insulating area where there is no contact between the members, with the isolated contact spots appearing as tiny conducting islands. The importance of this phenomenon in relationship to the present invention will be seen from the following discussion.
The typical dimension of a contact spot is of the order of one or two mils or less (1.0 mil equals 0.001"). These dimensions are determined by calculations using simplified models, as well as by measuring a quantity called (electrical) "constriction resistance", Rc. This quantity can be explained by referring to FIG. 1 depicting two imaginary cylinders J and K having their end surfaces finished as hemispheres which act as a pair of electric contacts and touch only at one contact spot. An electric current is passed from J to K in the direction of the arrows and is represented in the drawing by the series of lines. The lines of current are axial, uniform and straight except in the "constriction region" shown bounded by the two dashed lines. Within the constriction regions M, two phenomena are observed that contribute further instability to the contact spot as an electrical contact.
First, the lines of current have to bend to go through the contact spot which results in a longer path for the current to travel and, therefore, contributes an added resistance, known as construction resistance. The smaller the contact spot in relation to the diameter of the cylinders, the greater is the constriction resistance, which is several times larger than the bulk resistance of that portion of the conductor which contains the constriction.
Most of the heat flux from J to K, or vice versa, is carried thru the contact spot. A general rule that applies to metallic conductors is: heat flux and electric current are carried predominantly by the same electrons and, therefore, along the same path. This fact adversely affects the stability of these contact spots.
The second phenomenon associated with the above constriction is the appearance of an "electrodynamic blow-out force". This force tends to blow the contact members apart. Referring to FIG. 1, this force acts to push cylinder J up and cylinder K down, with the net effect of reducing the contact force, which increases the constriction resistance, increases the Joule heat I.sup.2 Rc, and makes the contact spot unstable. This force is encountered in all high current applications where current constriction takes place, or where current has a horizontal component parallel to the contact surface. In such a case the current sees a perpendicular magnetic field component which results in this force. The magnitude of the electrodynamic blow-out force is proportional to the square of the electric current passing thru the contact spot (constriction) and inversely proportional to the size of the spot.
A central fact relating to the prior art titanium drum cathode designs which use the shrink-fitting method to generate contact forces between top and base cylinders is that the size of the individual contact spots is too small compared to the axial thermal expansions and contractions of the top and base cylinders. For example, consider a typical drum: 60 inches wide, with a shrink-fitted titanium top cylinder, and a copper or stainless steel base cylinder. If this drum is taken from a room temperature ambient of 70.degree. F. and placed in a plating solution at 160.degree. F., there is a very large differential in thermal expansion between the top cylinder and the base cylinder. The temperature coefficients of linear expansion (TCLE), for titanium and copper are 4.6.times.10.sup.-6 and 9.2.times.10.sup.-6 inches per inch per degree F., respectively. Therefore, the copper base cylinder will expand axially by 60.times.9.2.times.10.sup.-6 .times.90=0.04968 inches. The titanium cylinder on the other hand will expand by 60.times.4.6.times.10.sup.-6 .times.90=0.02484 inches, which is nearly half the expansion of the base and about an order of magnitude larger than the size of the average contact spot.
This means that every time the drum is taken out of production, or put back in, most of the previous set of contact spots are replaced with a new set. Due to constriction resistance and the spot's small thermal inertia, the typical contact spot runs considerably hotter than the bulk material and is, therefore, at least partly oxidized. But, the oxidation process is accelerated following each thermal expansion or contraction, as this process exposes a freshly heated and unoxidized portion of a contact spot to air. If such a spot becomes a contact point in a subsequent expansion or contraction it would be more oxidized than the time before, its constriction resistance would be greater and it would operate at a higher temperature. If this contact spot survives the first expansion or contraction, its chances to survive the next would thus be diminished. This sequence of events occur in a "thermal runaway" situation.
As more and more of these contact spots are eliminated and become insulators, the remaining ones are left to operate at a higher current density. This, in turn, increases the blow-out forces, the operating temperature and the rate of oxidation, causing yet further deterioration of the remaining spots. When, due to this attrition of contact spots, the current density at the surviving spots exceeds a critical value, hot spots appear and failure of the drum follows in short order. In drums having a titanium top cylinder, this sequence of events, culminating in drum failure, has commonly occurred over a span of several months, limiting the drum life to less than a year.
Previously proposed solutions, as disclosed in U.S. Pat. Nos. 3,461,946 and 4,240,894, in the form of a base cylinder, or underdrum, made out of a material with higher LTCE than titanium failed to compensate for the resulting axial direction instability of the contact surfaces. In fact, these proposals, while helping the radial instability, made the axial instability worse.
Mild steel has been used for the underdrum because it has a higher LTCE than titanium. The use of this metal, however, introduces yet further undesirable complications. One such undesirable effect is the extra heat generated by eddy currents in the steel base sheet. Although, normally eddy currents are not a problem in direct current applications, they are in this application because the drum cathode rotates and only the immersed segment of the drum carries current. These factors produce a rate of change of magnetic flux which induces a certain EMF (electromotive force) in the steel base which, in turn, produces the eddy currents. These eddy currents and the heat they produce increase with current and the speed of rotation of the drum. The drum's speed, on the other hand, is keyed to the value of load current and the gauge of electrodeposited foil being produced.
Even higher contact pressure than provided by the shrink fitting technique has been proposed. In U.S. Pat. No. 4,240,894 there is proposed a series of raised portions on the supporting base drum thus presenting a smaller load bearing area. This might postpone, but will not reverse, the inevitable drum failure, because it does not solve the basic problem of contact spot instability as shown in the above analysis. Furthermore, the contact force between the titanium cylinder and the base cylinder, or supporting drum, is useful only up to the elastic limit irrespective of the source of the force. Beyond the elastic limit any additional force is dissipated through the production of plastic deformation and will not thus contribute any increase to the contact force.
There have also been proposals for introducing a soft sheet of copper or lead between the top titanium cylinder and the base cylinder. Such proposals are based upon the idea that a relatively soft and ductile metallic sheet would continuously deform to maintain "good contact" between the top titanium cylinder and the base cylinder. Not only does this arrangement fail to recognize and solve the basic instability problem, but it actually introduces an additional contact interface with the same problems. In fact, with this arrangement, one would have two constriction resistances in series: one between the titanium cylinder and the ductile sheet and the other between the ductile sheet and base cylinder.
The foregoing analysis and observed field experience lead me to the conclusion that a titanium drum design that relies on the shrink-fitting scheme or any source of mechanical force to provide an "electric contact" to carry suitable high currents between the titanium cylinder and base drum is an unstable, unreliable and low output design.
The present invention was developed as the result of efforts to produce an alternative titanium drum cathode construction free of the above problems and capable of operating at high currents up to and even exceeding 100 kiloamps (KA).
The primary object of the present invention is to provide an improved and reliable titanium drum cathode which has a titanium top cylinder on a supporting base cylinder which has a long service life free from the formation of hot spots.
Another object of the present invention is a drum cathode which has an increased current carrying ability and permits an increased rate of copper foil production.
Still another object of the present invention is an improved drum cathode which, when used in the production of copper foil permits, the foil to be produced with a minimum of surface imperfections and a more uniform weight distribution.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.